The present invention relates to a sintered electrode used in a discharge tube.
In general, a discharge tube such as a flashing discharge lamp, an arrester tube, a quenching tube and so on is provided with a pair of electrodes, at least one of them having electron emissive material in the metal body. The discharge tube is filled with inert gas such as xenon. The tube has a discharge characteristic determined by the distance between the electrodes, the envelope diameter, the kind of sealed gas and the pressure thereof. Typically, discharge tubes are used as light sources for photographs, strobes and light sources for preventing overcurrent in automatic light controlled devices.
In recent years, discharge tubes have been miniaturized, e.g., photoflash tubes for cameras. Consequently, the electrodes installed within the small space provided by the envelope of these miniaturized discharge tubes must have increased thermal resistence and an anti-ion bombardment property. Tungsten, molybdenum, tantalum and niobium have been used to form these electrodes since these metals are high melting point metals. These electrodes further contain electron emissive materials such as alkaline earth metal compounds and alkali metal compounds.
A typical electrode used in discharge tubes is a sintered electrode manufactured by the steps of compacting, compressing and sintering a high melting point metal powder with electron emissive material. It is also common to add a powder made of low melting point metal such as nickel and cobalt to improve sintering. Also, in order to purify gases within the envelope, a gas getter is often added to the electrode. The gas getter may be a metal such as a barium-aluminum alloy, titanium or zirconium.
In recent years, the sintered electrode used in miniaturized discharge tubes has been made of metals comprising tungsten as a main component, an additive for sintering, such as nickel, and an electron emissive material. An electrode made of these metals is suitable for a discharge tube operating with a relatively small current. On the other hand, when the electrode is used for a discharge tube discharging instantaneously large currents, such as a photo-flash tube, blackening of the tube occurs and the life of the tube is reduced because the nickel, which is necessary for easy sintering, evaporates from the electrode. Moreover, the starting voltage of the discharge tube is affected by undesired impurity gas created within the envelope. As a result, upon repeated discharges, the starting voltage increases. Although the undesired gas can be removed by disposing a gas getter within the envelope, the limited space within the envelope of a miniaturized discharge tube makes it difficult to include a gas getter.